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Why isn't x = 4 and y = 15 a valid solution if it satisfies the equation?
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luizabvillela

Bunuel
Official Solution:
Bunuel

A teacher distributes a batch of 56 cookies among \(x\) students such that each student receives exactly \(y\) cookies, where \(y > 0\), with some cookies left over. If these leftover cookies were redistributed, all except eight students would receive one additional cookie.

Select for x the possible value of \(x\), and select for y the possible value of \(y\) that would be jointly consistent with the given information. Make only two selections, one in each column.



xy
2
3
4
15
16
32

The number of cookies initially distributed was \(xy\).

Since redistributing the leftover cookies would give all except 8 students an additional cookie, the number of leftover cookies must have been \(x - 8\). This also indicates that there were more than 8 students in the group, so \(x > 8\).

Thus, we get \(xy + (x - 8) = 56\), which simplifies to \(x(y + 1) = 64\). This implies that \(x\) must be a factor of 64. The factors of 64 greater than 8 are 16, 32, and 64.

If \(x = 16\), then \(y = 3\).

If \(x = 32\), then \(y = 1\).

If \(x = 64\), then \(y = 0\), which is not possible since we are given that \(y > 0\).

Therefore, the possible pairs \((x, y)\) are \((16, 3)\) and \((32, 1)\). Among these pairs, only \((16, 3)\) is included in the options.


Correct answer:

x "16"

y "3"

Why isn't x = 4 and y = 15 a valid solution if it satisfies the equation?

While the pair x = 4 and y = 15 does satisfy the equation 4 * (15 + 1) = 64, it violates the condition that the number of leftover cookies is \(x - 8\).

If x = 4, then x - 8 = -4, which means there would be negative leftover cookies, which is not possible. The condition also implies that \(x > 8\), so x = 4 is invalid.
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I like the solution - it’s helpful.
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